Dual energy CT utilises two separate energy sets to examine the differing attenuation properties of matter, having a significant advantage over traditional single energy CT. Independent attenuation values at two energy sets can create virtual non-contrast images from contrast enhanced imaging as well as delineate the composition of renal calculi and arterial plaque 13.
There are three different dual energy technologies available:
- dual source dual energy
- two x-ray tubes producing different voltages offset at 90 degrees
- single source dual energy
- a single x-ray tube with fast switching voltage otherwise known as kVp switching
- single source dual layer
X-ray photons primarily interact with matter via the photoelectric effect, and Compton scattering producing the diagnostic images used in medicine today.
When an atom undergoes the photoelectric effect, the electron from that respected K-shell otherwise referred to as the inner shell is ejected via the incident photon. As that electron is excited, vacant space is 'filled' by a neighbouring electron, releasing energy as a photoelectron.
In short, when a photon has sufficient energy to overcome the electron's binding energy in the K-shell, that atom undergoes the photoelectric effect.
Each substance owns a unique K-shell binding energy; known as the K-edge. There is a significant spike in attenuation that results just beyond the energy of the K-edge, this peak is unique to every material and holds valuable information about the substance's composition.
The different photoelectric energies and K-edges are the bread and butter of dual-energy CT. Although most elements in the human body have very low K-edges (0.01-0.53 keV) elements like iodine and calcium, have a K-edge of 33.2 keV and 4.0 Kev respectively, making them sufficiently larger than surrounding structures and especially important in the clinical setting 1-3.
For instance, at 80 kVp a structure that contains no (introduced) iodine i.e. the liver has an attenuation based on its K-edge of x, yet when iodine (33.2 keV) is introduced into that same structure, it has a higher attenuation of y bringing it closer to 80 kVp.
As 80 kVp is closer to 33.2 keV than 140 kVp, the structures containing iodine will retain less attenuation as the kVp progressed beyond the K-edge of iodine. Therefore, when using two energies, it is possible to delineate structures based solely on their attenuation differences between 80 kVp and 140 kVp.
A dual x-ray source, tube A (140 kVp) and tube B (80 kVp or 100 kVp) with an angular offset of 90 degrees are preferred offsets for a dual source scanner in the current literature 1-5.
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